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Close up - The Aerogenerator X a new take on vertical-axis

Views on the viability of large vertical axis wind turbines are mixed to say the least. Now, one British group of companies is attempting to convince the doubters with the Aerogenerator X, a 10MW wind turbine concept designed for offshore use. Windpower Monthly technical writer Eize de Vries investigates.

The Aerogenerator X is an unusual and smart-looking vertical-axis turbine and functionally a Darrieus design. The distinct V-shape rotor features a double winglet at each blade tip while the proposed rotor diameter is 270 metres, more than double the size of any horizontal-axis turbine on the market. Due to their V-shaped-design, the two blades measure over 160 metres from tip to centre. The installation's total height when measured from the sea surface is about 130 metres.

The design

The concept follows an 18-month feasibility study dubbed NOVA. It was conducted by Wind Power Limited, Cranfield University, consultancy firm QinetiQ, Strathclyde University, Sheffield University, and independent consultants. Wind Power Limited is the company that developed the concept and will commercialise it through all project stages.

A long-term objective is that Aerogenerator series turbines will provide 1GW of UK offshore wind capacity by 2020.

Wind Power recently announced that it is in process of placing a contract with leading British consultancy firm Arup to provide dedicated project and product development support.

With the initial concept, both blades were for structural reasons interconnected by three horizontal guy wires attached at a different height along the blade's axis. Each blade was further fitted with four perpendicular evenly spaced "rigid sails" positioned along the component axis. These rigid sails acted as rotor stabilisers and also contributed towards optimising rotor blade aerodynamic performance.

Speaking about the design Wind Power's founder and managing director Theo Bird said: "The Aerogenerator X is twice the power and half the weight of our original design idea."

"The initial concept idea comprised a fixed structure without a tower, whereas the current design can be either a fixed or floating support structure. Another difference is the reduction of rigid sails and cross cables as part of the concept optimisation effort."

Advantages

Like classic 'eggbeater-shape' and 'H-shape Darrieus' wind systems, the Aerogenerator X rotor operates according to the aerodynamic lift principle, with the rotor swept area representing a triangular shape. Inherent to their working principle Darrieus-type wind systems are regarding aerodynamic efficiency at a disadvantage compared to conventional horizontal axis wind turbines.

But the main advantage of vertical-axis rotors is they can capture wind from all directions eliminating the need for a yaw system. And because the blades are not rising and falling against gravity they can be large, big, tough and easily maintained according to Bird. He says: "These characteristics together turn the Aerogenerator X concept into an ideal solution for the offshore environment."

Modern horizontal-axis three-blade wind turbines operate with pitch-controlled variable speed. As a built-in fail-safe measure each blade can act as an independent aerodynamic brake capable to bring the installation to a safe stop in case of an emergency.

Challenge

One of the toughest challenges for all large vertical-axis wind turbines is the design of an effective and fail-safe power output control. This is especially crucial when such installations are fitted with fixed-angle stall-type blades. The Aerogenerator X rotor turns at a maximum speed of around 3 rpm. The rotor torque is therefore huge and has to be controlled within relatively tight operational margins, including when it operates during extreme weather conditions like heavy storms.

Bird said that Wind Power is considering two possible alternatives to this problem. He said: "A likely option is variable speed operation in combination with fixed-angle stall-type blades and electronic output control. However, we do not rule out active power output control via pitch able blades. But our preference is always the simplest solution as health and safety and environmental issues are all of key importance in the challenging marine environment."

Bird is confident about the benefits of his Aerogenerator X concept for the fast growing offshore wind market. He said: "Conventional horizontal axis wind turbines offer a brilliant and proven solution for multiple onshore applications. However, for large-scale future offshore wind plants I am convinced that our Aerogenerator X solution offers unbeatable added value in terms of uncomplicated installation and operation with the lowest Cost of Energy performance."

Verdict

However, once the turbine has been designed and field-tested, Wind Power's next task is to persuade the risk-adverse banks that the new and relatively untried design can decade-long perform in the tough offshore environment. The 10MW capacity leap is important but arguably, the best and most wanted asset for any offshore wind turbine is reliability. This explains why the Siemens 3.6MW 'workhorse' with its offshore strong track record is still doing so well, despite the bigger alternatives out there.

But on the other hand there can't be progress without heading into unknown waters. If we were risk-adverse, we'd still be using horses for transport and communicating via the post only.

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